1. Field of the Invention
The teachings herein relate in general to the detection of ionizing radiation and more particularly to spectroscopic fast neutron detection and discrimination using Li-based semiconductors.
2. Description of the Related Art
As shown in FIG. 3, the conventional energy domains for neutrons are labeled as: (i) thermal neutron energy (En=2.5 E-8 MeV); (ii) epithermal neutrons (2.5 E-8 MeV<En<1 e-6 MeV); (iii) resonance neutrons (1 e-6 MeV<En<0.01 MeV); and (iv) fast neutrons (En>0.01 MeV). FIG. 4 shows the energy region of interest (“fast neutrons”) to better show the challenge in measuring with sufficient accuracy the neutron energy spectra of different neutron sources. In addition to fast neutrons arriving directly from their source, the detectors used in field applications must discriminate the following types of radiation: (A) fast neutrons generated by cosmic rays, (B) neutrons from the source that have lost energy by collisions with nearby shielding, ground, bodies of water, buildings, etc., and (C) gamma rays from the natural background covering a range of energies up to 3 MeV (Egamma), as shown in FIG. 5.
Neutron sensing has been addressed in the past using scintillating material, ionizing gas materials and diffused solid material or a combination of any of these. The scintillating materials usually include elements presenting natural isotopes with large neutron reaction cross sections, which, upon absorbing a neutron, decay into charged particles. These charged particles lose energy within the scintillator and produce light intensities proportional to the energy of the incident neutron. The ionizing gas materials contain neutron-absorbing isotopes in a gaseous form under a large voltage gradient. Neutrons absorbed in the gas cause nuclear decay in the gaseous material and the decay products are accelerated towards one of the high voltage electrodes. The signal emitted by each electrode is proportional to the energy of the incident neutron. The diffused solid material uses small concentrations of neutron absorbing isotopes diffused into a semiconductor material. Upon capture of the incident neutron, the neutron-absorbing isotope decays and its charged decay products produce current pulses in the semiconductor material which are proportional to the incident neutron energy.
Improving the energy resolution for the scintillator-based approach has been addressed in the past by improving the scintillating material composition and/or using more efficient photomultiplier tubes or photo diodes. The energy resolution in the ionizing gas neutron detectors has been improved in the past by using improved data acquisition electronics with optimal shaping times and pileup rejection circuitry.
A neutron detector with spectroscopic (i.e., energy measurement) capability should be efficient in capturing the neutrons and converting their energy into a physical quantity measurable with sufficient precision. At the same time, the detector must be relatively insensitive to gamma rays and capable of separating their signal from the neutron signal.
Fast neutron spectroscopy has mostly been applied in areas of basic research (nuclear physics and astrophysics) and has commercial applications in the nuclear energy domain (reactor operation, fuel and waste management) or for radiodosimetry (either for reactor personnel or neutron-based cancer therapy). Recently, it has been recognized that neutron detection can be a complementary technique to gamma ray spectrometry for Homeland Security applications. Mainstream commercial neutron detectors, however, perform only simple neutron counting.
Even though energy resolution for the diffused 6Li neutron detectors have been recently improved through increased 6Li loading and the use of better data acquisition electronics, none of the previous methods of data acquisition has the ability of achieving superior energy resolution.
What is needed is a spectroscopic fast neutron detection and discrimination detector that provides superior energy resolution for Homeland Security in applications such as passive identification of radioisotopes with spontaneous fission or active interrogation/verification.